SPECULOOS Southern Observatory (SSO)
The SSO is at the heart of a new exoplanet detection project called SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOOl Stars). It consists of four Ritchey-Cretien telescopes equipped with 1 m aperture mirrors. The telescopes, named Io, Europa, Ganymede and Callisto after the four Galilean moons of Jupiter, will enjoy perfect viewing conditions at the Paranal site in Atacama Desert of Chile, where the VLT (Very Large Telescope of ESO is also located. Paranal offers an almost perfect site for astronomy, with a dark sky and a stable and arid climate. SPECULOOS will focus on detecting Earth-sized planets orbiting nearby ultra-cool stars and brown dwarfs. 1)
The SPECULOOS Southern Observatory is a project carried out by the University of Liège (Belgium) and the Cavendish Laboratory in Cambridge (United Kingdom), under the direction of Michaël Gillon, researcher at the head of the "EXOplanets in Transit: Identification and Characterization" (EXOTIC) group within the Department of Astrophysics, Geophysics and Oceanography at the University of Liège. ESO (European Southern Observatory) supports SPECULOOS South and hosts it at the Paranal Observatory located in the Chilean Atacama Desert. SPECULOOS South is mainly funded by the ERC (European Research Council).
The SPECULOOS project aims to search for planets the size of the Earth that could potentially be inhabited around ultra-cool or brown dwarfs stars. Despite that there are some examples of exoplanets orbiting these stars, they represent only a tiny fraction of all exoplanets discovered, and even less are potentially habitable. Although these small stars are more difficult to observe, they are abundant (they represent about 15% of the stars in our galaxy), and SPECULOOS is designed to explore the nearest 1000 stars, the brightest and smallest, and thus detect Earth-like planets in their habitable zone.
SPECULOOS will search for these exoplanets by the so-called transit method. When a planet passes in front of its star, it blocks part of the star's light, causing a small partial eclipse and a subtle but detectable attenuation of the star's light. Exoplanets with smaller host stars block more of the light from their stars during transit, making these periodic eclipses easier to detect.
So far, only a small fraction of the exoplanets detected by this transit method have been found to be the size of the Earth or smaller, due to the limitations of telescope observation specifically dedicated to this detection. However, the SPECULOOS project could lead to new discoveries of planets in this size range, both because of the proximity and small size of the stars targeted by the project, and the relatively large diameter of the telescope mirrors.
"These new telescopes will allow us to detect not only potentially habitable exoplanets around nearby stars, but also exoplanets that can then be studied in great detail, especially to look for chemical traces of life. This is an extremely exciting time for the science of exoplanets." concludes Michaël Gillon.
Figure 1: The telescopes of the SPECULOOS Southern Observatory gaze out into the stunning night sky over the Atacama Desert, Chile (image credit: ESO, P. Horálek) 2)
One of the most fundamental questions raised by humankind is how frequently, and under which conditions life exists around other stars. Its scientific answer requires the detailed atmospheric characterization of temperate rocky exoplanets to search for potential biosignature gases. 3) The sample of transiting exoplanets found so far represents a genuine Rosetta stone to understand extrasolar worlds because their special geometry offers the possibility to study their atmospheres through eclipse (transit and occultation) spectroscopy, without having to spatially resolve them from their host stars. 4)
Over the last ~15 years, these techniques have been applied to transiting hot Jupiters, Neptunes, and a few favorable Super-Earths, giving us first insights into their atmospheric chemical compositions, pressure-temperature profiles, albedos, and circulation patterns (for comprehensive reviews. However, exporting these methods to the atmospheric characterization of an Earth-twin transiting a Sun-like star is out of reach for all the existing or currently planned astronomical facilities. The main reasons for this are the lack of a suitable target, the overwhelmingly large area contrasts between the solar disk and the tiny Earth’s atmospheric annulus, and between the Sun’s and Earth’s luminosities. They lead to very-low signal-to-noise ratios (SNRs) for any atmospheric spectroscopic signature and any realistic observing program with upcoming facilities such as the James Webb Space Telescope. Fortunately, this negative conclusion does not hold for an Earth-sized planet transiting a nearby host of the smallest kind - a nearby UCD (Ultra-Cool Dwarf).
Figure 2: Left: Probability of transit (dotted blue), transit depth (dashed red), and number of orbits (i.e. number of transits or occultations) per year (black) of an Earth-sized planet with an equilibrium temperature of 255 K (like Earth) as a function of the host’s mass. Right: Ratio (Bλ(T*))1/2/(R*) – normalized for a Sun-like star – as a function of the effective temperature T*. For a given planet (fixed radius, mass, and equilibrium temperature), the SNR of an atmospheric spectral feature in transmission scales as this ratio.
Of course, these UCD targets are spread all over the sky, which means that they have to be monitored individually. However, the short orbital periods of planets in the habitable zone around UCDs translate into a required photometric monitoring of much smaller duration for the detection of such planets than for Earth-Sun twin systems. Consequently, a transit search targeting the ~1200 UCDs of our sample could be done within ~10 years with a realistically small number of ground-based telescopes. Still, this strategy makes necessary being able to detect a single transit event to prevent allocating one telescope to one UCD for unrealistic durations, and thus results in a stringent requirement for very-high photometric precisions (≤ 0.1%).
Due to their low temperatures, UCDs are faint in the optical, their spectral energy distribution peaking at near- and mid-IR wavelengths. Using large telescopes combined with infrared detectors could thus appear a priori as the only option for achieving high photometric precisions on these objects. SNR computations convinced us that it was not the case, and that 1m-class telescopes equipped with near-IR-optimized CCD cameras (providing good efficiencies up to 1 µm) should reach the required high photometric precisions.
The SPECULOOS project
SPECULOOS started back in 2011 as a prototype mini-survey on our TRAPPIST-South telescope, a 60 cm Ritchey-Chretien telescope that has been installed since 2010 at ESO La Silla Observatory in the Chilean Atacama Desert. The telescope is equipped with a 2k x 2k thermoelectrically-cooled back-illuminated CCD camera offering excellent quantum efficiencies from 300 to beyond 900 nm. It has a field of view of 22’ x 22’ and a pixel scale of 0.64”/pixel. This prototype survey targeted 50 among the brightest southern UCDs (Ultra Cool Dwarfs), with K-magnitude between 5.3 and 11.4 (mean K-mag=9.7), and uniformly distributed in terms of spectral type and sky position. Its concept is to monitor in a wide near-IR filter (transmittance > 90% from 750 nm) each UCD during at least 100 hours spread over several nights. The initial goals of this prototype survey were to assess the typical photometric precisions that can be reached for UCDs on nightly timescales with such an instrumental setup and the resulting detection thresholds for terrestrial planets. 5)
UCDs have characteristics that make them not only ideal targets to search for transiting temperate rocky worlds, but also optimal for their atmospheric characterization. Figure 2 (left) shows the probability of transit, transit depth, and number of orbits per year for an Earth-sized planet, with an equilibrium temperature of 255 K (like Earth), as a function of the host’s mass. It can be seen that UCDs present several advantages. First, their small size leads for Earth-sized planets to transit signals that are about two orders of magnitude deeper than for Earth-Sun twin systems. Expected transit depths range from a few 0.1% up to >1%, which is routinely detected by many ground-based surveys. 6)
Results: About 40 UCDs were observed by TRAPPIST-South in the period from 2011 to 2017. Half of the observed UCDs show “flat” light curves, i.e. stable photometry on the night timescale (Figure 3 - top). Some of the other UCDs (~20%) show clear flares in some light curves. These flares are seen in near-IR light curves as sudden increase of a few percent of the measured brightness, followed by a gradual decrease back or close to the normal level, the whole process taking only 10 to 30 min (Figure 3 - middle).
In the context of a transit search, it is easy to identify and discard the portions of light curves affected by flares. Furthermore, their frequency is relatively small (1 flare per 3-4 nights on average). Finally, about 30% of the observed UCDs show some rotational modulation (and more complex variability) with up to 5% amplitude (see Figure 3 - bottom). These rotational modulations do not limit the ability to detect transits as they can be modeled and corrected. One of these objects is the nearby brown dwarf binary Luhman 16AB, that we monitored for nearly a fortnight as part of our prototype survey, right after its discovery was announced in February 2013. 7) The quality of our photometric data allowed us to reveal fast-evolving weather patterns in the atmosphere of the coolest component of the binary, as well as to firmly discard the transit of a two-Earth-radii planet over the duration of the observations and of an Earth-sized planet on orbits shorter than ~9.5 hours. 8)
Figure 3: Typical TRAPPIST-South light curves of UCDs obtained as part of our prototype survey. We consider 3 different UCDs and present for each of them 3 light curves, obtained over different nights. For each light curve, the measurements are shown unbinned (cyan points) and binned per 7.2 min (black points with error bars). The first target (top) shows rather flat light curves. The second one (middle) shows clear flares in some light curves (see the first and third light curves shown here). The last target (bottom) shows some rotational modulation with an amplitude of ~4% and a period of ~2.8 hours.
From simulations based on the injection and recovery of synthetic transits of terrestrial planets in TRAPPIST-South UCD light curves, we have reached the conclusion that the variability of a fraction of UCDs (flares and rotational modulation) does not limit the ability to firmly detect transits of close-in Earth-sized planets. The achieved photometric precisions are globally nominal, with no hint of extra correlated noise, except when the observations were performed in high-humidity conditions. Near-IR ground-based CCD time-series photometry of UCDs from a suitable astronomical site (good transparency, low humidity) can thus reach nominal sub-mmag precisions. These conclusions were recently strengthened by our detection of an amazing planetary system around one of the TRAPPIST-South UCD target, TRAPPIST-1. 9)
The nearby (~12 pc) TRAPPIST-1 system is composed of a middle-aged M8-type dwarf star orbited by seven nearly Earth-sized57 planets in orbits of 1.5 to 19 days. All seven planets are temperate (equilibrium temperatures <400 K assuming a null Bond albedo) and the three planets TRAPPIST-1e, f, and g orbit in the habitable zone around the star. Remarkably enough, the seven planets form a (near-)resonant chain, with all sets of three adjacent planets in Laplace three-body resonances. This particular configuration greatly enhances the gravitational interactions between the planets and the resulting transit timing variations, whose dynamical modeling makes it possible to constrain the bulk densities and eccentricities of the planets. The TRAPPIST-1 planets have thus become prime targets for the study of temperate terrestrial worlds outside the Solar System, including the study of their atmospheres, owing to their transiting nature, combined with the infrared brightness (K-mag=10.3), Jupiter-like size , and low luminosity of their host star.
Status and some observational imagery of SPECULOOS
• March 9, 2020: An international team of scientists including researchers from the University of Liège has just identified an eclipsing binary brown dwarfs with the telescopes of the SPECULOOS project led by Michaël Gillon, researcher at the University of Liège. This rare discovery - only one other binary system of this type had been discovered more than 10 years ago - has just been published in the journal Nature Astronomy. 10) 11)
Figure 4: Artistic view of WISE 0855 as it might appear if viewed up close in infrared light (image credit: Joy Pollard, Gemini Observatory/AURA)
- An international team of scientists, led by Amaury Triaud, a researcher at the University of Birmingham, has just made an important discovery: an eclipsing binary system composed of two brown dwarfs in eclipse, one of them passing in front of the other at each orbit. Brown dwarfs are so-called "sub-stellar" objects, they form like stars but have too little mass to allow nuclear fusion in their core, the process that characterizes normal stars. Although scientists believe that brown dwarfs are widespread in the universe, they are still difficult to detect because of their low luminosity. In this case, the observations were obtained shortly after the construction of the first telescopes, while they were still in the test phase," explains Michaël Gillon, astronomer at the University of Liège (Astrobiology / Faculty of Science), initiator and principal investigator of the SPECULOOS project (Search for habitable Planets EClipsing ULtra-cOOl Stars), at the origins of the discovery. We had turned one of our telescopes in Chile toward a known brown dwarf (2MASSW J1510478-281817) and observed a decrease in brightness of this object for about 90 minutes, indicating that an eclipse had just occurred." A signal that enabled the researchers to conclude to the binary nature of the brown dwarf.
- We were able to confirm our hypothesis by using two more powerful telescopes: the Keck, a 10 m diameter telescope located in Hawaii and the VLT (Very Large Telescope) of 8 m located in Chile, a few kilometers from our SPECULOOS-South Observatory," says Laetitia Delrez, a researcher at ULiège (Astrobiology and STAR Institute). Observations of eclipsing binary brown dwarfs are extremely rare - only one other similar system has been identified, more than 10 years ago. "This configuration will allow astronomers to measure their radius and mass without making assumptions, they have already been able to measure the individual velocities of the two brown dwarfs, allowing them to constrain their masses. "In addition to their radii, obtained from the eclipse and luminosity, we were also able to estimate their age - around 50 million years-, which is really rare because usually in the measurements of these objects, at least one other element is missing," explains Dr Amaury Triaud, researcher at the School of Physics & Astronomy at the University of Birmingham, who led the analysis.
- The team that produced the discovery consists of researchers from the University of Birmingham, the University of California, San Diego, the University of Liège, the Institute of Astrophysics of the Canaries, the American Museum of Natural History, the University of Cambridge, the University of Bern, the Massachusetts Institute of Technology, the University of Göttingen and the University of Warwick.
• December 5, 2018: The SPECULOOS Southern Observatory (SSO) has been successfully installed at the Paranal Observatory and has obtained its first engineering and calibration images — a process known as first light. After finishing this commissioning phase, this new array of planet-hunting telescopes will begin scientific operations, starting in earnest in January 2019 (Ref. 2).
SSO is the core facility of a new
exoplanet-hunting project called SPECULOOS (Search for habitable
Planets EClipsing ULtra-cOOl Stars), and consists of four telescopes
equipped with 1-meter primary mirrors. The telescopes — named Io, Europa, Ganymede and Callisto after the four Galilean moons of Jupiter — will enjoy pristine observing conditions at the Paranal site, which is also home to ESO’s flagship Very Large Telescope (VLT). Paranal provides a near-perfect site for astronomy, with dark skies and a stable, arid climate.
These telescopes have a momentous task — SPECULOOS aims to search for potentially habitable Earth-sized planets surrounding ultra-cool stars or brown dwarfs, whose planetary populations are still mostly unexplored. Only a few exoplanets have been found orbiting such stars, and even fewer lie within their parent star’s habitable zone. Even though these dim stars are hard to observe, they are abundant — comprising about 15% of the stars in the nearby universe. SPECULOOS is designed to explore 1000 such stars, including the nearest, brightest, and smallest, in search of Earth-sized habitable planets.
“SPECULOOS gives us an unprecedented ability to detect terrestrial planets eclipsing some of our smallest and coolest neighboring stars,” elaborated Michaël Gillon of the University of Liège, principal investigator of the SPECULOOS project. “This is a unique opportunity to uncover the details of these nearby worlds.”
SPECULOOS will search for exoplanets using the transit method, following the example of its prototype TRAPPIST-South telescope at ESO’s La Silla Observatory. That telescope has been operational since 2011 and detected the famous TRAPPIST-1
planetary system. As a planet passes in front of its star it blocks
some of the star’s light — essentially causing a small
partial eclipse — resulting in a subtle but detectable dimming of
the star. Exoplanets with smaller host stars block more of their
star’s light during a transit, making these periodic eclipses
much easier to detect than those associated with larger stars.
Thus far, only a small fraction of the exoplanets detected by this method have been Earth-sized or smaller. However, the small size of the SPECULOOS target stars combined with the high sensitivity of its telescopes allows detection of Earth-sized transiting planets located in the habitable zone. These planets will be ideally suited for follow-up observations with large ground- or space-based facilities.
“The telescopes are kitted out with cameras that are highly sensitive in the near-infrared,” explained Laetitia Delrez of the Cavendish Laboratory, Cambridge, a co-investigator in the SPECULOOS team. “This radiation is a little beyond what human eyes can detect, and is the primary emission from the dim stars SPECULOOS will be targeting.”
The telescopes and their brightly colored mounts were built by the German company ASTELCO and are protected by domes made by the Italian manufacturer Gambato. The project will receive support from the two TRAPPIST 60-cm telescopes, one at ESO’s La Silla Observatory and the other in Morocco . The project will in due course also include the SPECULOOS Northern Observatory and SAINT-Ex, which are currently under construction in Tenerife, Spain, and at San Pedro Mártir, Mexico, respectively.
There is also potential for an exciting future collaboration with the Extremely Large Telescope (ELT), ESO’s future flagship telescope, currently under construction on Cerro Armazones. The ELT will be able to observe planets detected by SPECULOOS in unprecedented detail — perhaps even analyzing their atmospheres.
Figure 5: This first light image from the Europa telescope at the SPECULOOS Southern Observatory (SSO) shows the heart of the Carina Nebula (image credit: SPECULOOS Team/E. Jehin/ESO) 12)
• February 22, 2017: The discovery of the Trappist-1 system was made in the context of SPECULOOS. After this first discovery, SPECULOOS aims to detect more systems of this type, thanks to four telescopes currently being installed on the European Southern Observatory of Paranal (ESO) in Chile that will be able to observe more targets than this prototype. According to Michaël Gillon, “SPECULOOS, which will observe ten times as much targets and with greater precision, should detect many more, placing itself at the frontline of research into the search for life elsewhere in the Universe” 13) 14) 15)
A milestone was recently achieved with the detection of three Earth-sized planets transiting (i.e. passing in front of) a star just 8% the mass of the Sun 12 parsecs away. Indeed, the transiting configuration of these planets combined with the Jupiter-like size of their host star - named TRAPPIST-1 - makes possible in-depth studies of their atmospheric properties with current and future astronomical facilities.
Here we report the results of an intensive photometric monitoring campaign of that star from the ground and with the Spitzer Space Telescope of NASA. Our observations reveal that at least seven planets with sizes and masses similar to the Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.21, 12.35 days) are near ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inward. The seven planets have equilibrium temperatures low enough to make possible liquid water on their surfaces.
Figure 6: Five of the seven planets orbiting the dwarf star have been discovered in 2016 with the ground-based TRAPPIST telescope located in La Silla, Chile. The seven planets of TRAPPIST-1 are all Earth-sized and terrestrial, according to research published in 2017 in the journal Nature. TRAPPIST-1 is an ultra-cool dwarf star in the constellation Aquarius, and its planets orbit very close to it (image credit: TRAPIST-1 Team, NASA, R. Hurt/T. Pyle) 16)
Legend to Figure 6: This artist's concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets' diameters, masses and distances from the host star. The system has been revealed through observations from NASA's Spitzer Space Telescope and the ground-based TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) telescope, as well as other ground-based observatories. The system was named for the TRAPPIST telescope.
The seven planets of TRAPPIST-1 are likely all tidally locked, meaning the same face of the planet is always pointed at the star, as the same side of our moon is always pointed at Earth. This creates a perpetual night side and perpetual day side on each planet.
TRAPPIST-1b and c receive the most light from the star and would be the warmest. TRAPPIST-1e, f and g all orbit in the habitable zone, the area where liquid water is most likely to be detected. But any of the planets could potentially harbor liquid water, depending on their compositions.
At about 40 light-years (235 trillion miles) from Earth, the system of planets is relatively close to us, in the constellation Aquarius. Because they are located outside of our solar system, these planets are scientifically known as exoplanets.
This exoplanet system is called TRAPPIST-1, named for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. In May 2016, researchers using TRAPPIST announced they had discovered three planets in the system. Assisted by several ground-based telescopes, including the European Southern Observatory's Very Large Telescope, Spitzer confirmed the existence of two of these planets and discovered five additional ones, increasing the number of known planets in the system to seven.
Using Spitzer data, the team precisely measured the sizes of the seven planets and developed first estimates of the masses of six of them, allowing their density to be estimated.
Based on their densities, all of the TRAPPIST-1 planets are likely to be rocky. Further observations will not only help determine whether they are rich in water, but also possibly reveal whether any could have liquid water on their surfaces. The mass of the seventh and farthest exoplanet has not yet been estimated – scientists believe it could be an icy, "snowball-like" world, but further observations are needed.
In contrast to our sun, the TRAPPIST-1 star – classified as an ultra-cool dwarf – is so cool that liquid water could survive on planets orbiting very close to it, closer than is possible on planets in our solar system. All seven of the TRAPPIST-1 planetary orbits are closer to their host star than Mercury is to our sun. The planets also are very close to each other. If a person was standing on one of the planet’s surface, they could gaze up and potentially see geological features or clouds of neighboring worlds, which would sometimes appear larger than the moon in Earth's sky.
• In early 2016, a team of astronomers, also led by Michaël Gillon announced the discovery of three planets orbiting TRAPPIST-1. They intensified their follow-up observations of the system mainly because of a remarkable triple transit that they observed with the HAWK-I instrument on the VLT (Very Large Telescope). This transit showed clearly that at least one other unknown planet was orbiting the star. And that historic light curve shows for the first time three temperate Earth-sized planets, two of them in the habitable zone, passing in front of their star at the same time! 18) 19)
Figure 7: This plot shows the varying brightness of the faint dwarf star TRAPPIST-1 during an unusual triple transit event on 11 December 2015. As the star was monitored using the HAWK-I instrument on ESO’s Very Large Telescope three planets passed across the disc of the star, each causing some of its light to be blocked. This historic light curve shows for the first time three temperate Earth-sized planets, two of them in the habitable zone, passing in front of their star (image credit: ESO, Michael Gillon et al.) 20)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).